The present invention relates to a method and apparatus for detecting decrease in tire air-pressure and a program for judging decompression of a tire. More particularly, it relates to a method and apparatus for detecting decrease in tire air-pressure and a program for judging decompression of a tire with which it is possible to detect decompression during initialization at an early stage and at high accuracy.
An apparatus for detecting decrease in tire air-pressure (DWS) in which decompression of a tire is detected on the basis of rotational information of four wheel tires attached to a vehicle is conventionally known. Such an apparatus employs a theory that a rotational angular velocity of a decompressed tire is increased when compared to remaining normal tires owing to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire of normal internal air-pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires (reference should be made to Japanese Unexamined Patent Publication No. 305011/1988),DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is employed as a judged value DEL. Here, F1 to F4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
It is judged that decompression of a tire is present when a value which is obtained by calculating the above judged value DEL and performing necessary corrections such as cornering correction and rejections of unnecessary data has exceeded a preliminarily set threshold.
When employing such a method of detection, it is necessary to perform initialization in a condition in which the air-pressure is normal and to learn differences in initial rotational angular velocities for compensating differences in rotational angular velocities due to manufacturing variations or other factors of tires. Presently, approximately 1.5 hours is a minimum required time for initialization. While it is possible that decompression occurs through burst or similar until the initialization is completed, such decompression of a tire during initialization cannot be detected.
It is possible to consider a method in which a threshold that is larger than a normal one is set for enabling detection of decompression of a tire and to accordingly issue alarm also during initialization to some extent.
However, while decompression might be detected also during initialization when the decompression due to a burst happens abruptly and the degree thereof is large, a drawback is presented that initialization might also be completed without issuing an alarm when the decompressing speed is slow.